The orgies – or "slamming parties", as they've been dubbed ("slamming" is a euphemism for "injecting") – are known to a small but rapidly expanding section of London's gay community. They are covertly organised over social networking sites like Grindr and Bareback Real Time.

However, that's not to say they're particularly exclusive; as long as you're gay, don't mind potentially exposing yourself to a host of STDs and can get into the idea of taking Viagra and injecting crystal meth (and sometimes mephedrone) for several days straight, you're welcome to swing through the revolving doors and join the party.

The various slamming get-togethers are pretty fluid affairs, taking place over several days and in several venues, from darkened private homes and West End saunas to dingy flats and suburban mansions. There are constants, however – the non-stop porn being streamed on massive projector screens, the cascading synthlines of shitty Eurotrance and the glow of guests twiddling with their iPhones, as they attempt to get hold of more drugs and bodies to invite along.

“This new scene of bare-backing and injecting is pushing the limits of what’s socially acceptable,” says Tim, a 39-year-old web publisher who hosted and attended slamming parties for two years before giving it up. “Injecting crystal meth makes you incredibly horny and willing to do anything. People turn into animals when they come up on it. It’s basically a blizzard of sucking and fucking.”

Yes, the scene might sound a bit like the more hedonistic, cosmopolitan equivalent of dogging; less Micras, more Mosley madness – Dogging: A Love Story reimagined by William S Burroughs and Tinto Brass. But, given the sharing of needles and lack of condoms, the repercussions that come from these blizzards of sucking and fucking are arguably much more grim than what you're left with after a stolen encounter in a Stevenage layby.

Specialist drug services are witnessing a rise in gay men addicted to injecting crystal meth and, more worryingly, a jump in gay drug users who are testing positive for HIV.

Someone injecting meth at a slamming party.

According to David Stuart, director of Antidote – London's only dedicated LGBT-specific drug and alcohol support service – the number of crystal meth and mephedrone users injecting the drugs in a sexual context leapt from 20 percent in 2011 to 80 percent in 2012. Seventy percent of those injecting are reportedly sharing needles. "It's a staggering and frightening increase," Stuart told me.

And, of course, what makes these slamming parties unique is the slamming itself. Injecting meth (or "Tina", as it's commonly referred to in the gay community) provides a far more intense, longer and therefore cheaper hit than smoking it, ramping up your libido and stripping inhibitions. It's turned London's already pretty athletic gay sex-party scene into an extreme sport, with revellers apparently averaging up to five sexual partners a session.

“The one thing I want to do on crystal is to get fucked by the biggest dick,” says Tim, whose teeth have rotted away due to a combination of being HIV positive and many years of injecting and smoking meth. “I was known as a total party bottom," he continued.

Some slamming parties are more extreme than others, with those at the harder end of the scale usually involving hardcore S&M, whipping and bondage. And if two people want to indulge in something others might not be comfortable with – like fisting or scat, for example – then they carry on the party elsewhere.

“People are often awake for days with no food or water, just fizzy drinks and Dunns River Nurishment [a nutritional milk supplement]," Tim told me. "But the stupid thing is that no one can ever cum, because crystal meth stops you cumming – as does Viagra – so it's just never-ending sex. It’s painful. Most people end up with no skin on their dicks and some end up in hospital because of panic attacks brought on by too much crystal,” he continued.

Tim says that, although he was one of the first to organise slamming and sex parties, the scene has become more widespread in the last couple of years. And many of the people now involved in bareback slamming are reportedly well-heeled professionals, despite the extreme nature of their drug use.

“There are those who pay for the drugs in order to attract parties. And, at the other end of the scale, there are people who are invited to parties because they're well hung and can get an erection on crystal with or without Viagra,” Tim told me, before recounting one of the parties he held a few years ago. “People came down from Manchester one time and there were about 12 guys coming in and out of my house. I remember my dark, sweaty living room with half a dozen men having sex with each other. Everyone else was checking out the internet for people in the area or squabbling over which porn stream to watch.”

Tim says the golden rules to holding these kind of parties are to hide your keys and your drugs, and to lock your doors, “otherwise your drugs will be gone and you'll have guests freaking out in the street”.

Victor – a 23-year-old Romanian who moved to London four years ago – has just finished treatment for crystal meth addiction after being involved with the slamming party scene.

“I had used cocaine and ecstasy before coming to England, but I met a dealer and he introduced me to lots of people. I tried smoking crystal meth and drinking GHB – I had great sex," Victor explained. “The first time I injected Tina was at a party in West London. Everyone was injecting and I tried it and it gave me an even bigger high. It was so incredible – I wanted that high again. I had no inhibitions, I tried new things, I got involved in sex parties – it was crazy.”

Both Tim and Victor know how deadly the slamming scene can be. Both have friends who have ended up in hospital, died or committed suicide, either because of the psychological effects of meth addiction or because they have contracted HIV or hepatitis C.

David Stuart told me that around 75 percent of the 800 men being treated at Antidote’s services are HIV positive, with 60 percent failing to adhere to their HIV treatment when under the influence of drugs. “Lots of things are driving [the drug use], including the ease of finding the drugs themselves and the use of internet sites to find sex parties and drug dens where people can carry out this behaviour,” he told me.

Stuart said the motives for getting involved in the scene are more complicated than pure hedonism. “Many gay men feel their sex is 'diseased' or 'sinful' – the kind their parents disapprove of. A culture of online 'hooking up' for sex is eradicating the usual process of developing an intimacy of sorts before having sex. Drugs can overcome these problems, too, providing an uninhibited abandon that these men rarely feel.”

Yusef Azad is the director of policy at the National AIDS Trust, which, as an organisation, has sent a letter to all London councils calling for action to address the lack of specialist services that address the "recent and rapid rise in the use of crystal meth in the context of high-risk sex".

Speaking to Azad, he explained the circumstances that prompted that letter: “What has changed is the sort of drugs that are used and the context in which they’re used. A lot of drugs are moving from clubbing to private sex parties. Apps like Grindr are facilitating networking among gay men for extended sex sessions on drugs. Everything we are hearing from clubbers and gay men on this scene is that it is prevalent and increasing. Three years ago, this wasn't mentioned at all.”

For both Tim and Victor, being involved in such an intense sexual scene has left them unable to have sex without the drugs. “It’s boring,” says Victor. “I can’t get horny without drugs. So for me now, I cannot give up drugs without giving up sex. It’s a been a huge waste and it's ruined my life.”

Max Daly is a journalist and author specialising in social affairs and illegal drugs. He is the co-author of Narcomania: A Journey through Britain's Drug World, published by Random House